Technical Field
This invention generally relates to methods and devices for applying solder to workpieces. More particularly, this invention relates to a method and an apparatus which uses a shield to allow selective application of solder to a circuit board with a wave soldering device.
Background
Most electronic devices use a circuit board as the fundamental building block for the electronic circuitry which forms the device. With the advent of the printed circuit board (PCB), the board itself actually became part of the circuit, forming conductors for the electrical current between the various electronic components. The circuit board both supports the electronic components and links them together. Generally, it consists of a nonconductive base, such as fiberglass, coated with a thin layer of a conductor, such as copper, which is etched to form a pattern of electrical conductors. Further advances in PCB technology resulting in dual sided PCBs, to which components are attached on both sides of the boards, and in multilayered PCBs, which allow intricate and complex circuits to be implemented in a smaller footprint. The various layers of multilayered PCB are electrically connected together at desired points by vias, which are vertical conductive shafts formed in the base of the board.
Soldering of the various electronic components to a circuit board is generally accomplished by one or more of the following processes: manual soldering; reflow soldering; batch soldering using a solder pot; or continuous soldering using a wave soldering device. Manual soldering involves employing a manufacturing technician using a soldering iron to solder each joint individually. This is an expensive and relatively slow manufacturing technique. However, it is sometimes necessary, especially when a particular component can't be soldered using one of the other soldering techniques, as might be the case where a component is heat sensitive and cannot be subjected to the high temperatures of reflow, pot and wave soldering.
Reflow soldering involves applying a paste of flux and solidified solder balls to the electrical connecting points on the component and the connection pads on the circuit board and heating the entire assembly in an oven to make the solder reflow to form an electrically conductive connection. This process requires that all of the components be able to withstand the relatively high temperatures necessary to form a reliable solder joint. Additionally, while well suited to mounting surface mount components, this process is not suited for attachment of through hole components.
Solder pot soldering involves batch processing, usually a single board at a time. A solder pot is a device which has a reservoir of molten solder, a plenum extending from the reservoir, one or more flow wells attached to the plenum and a pump for pumping the solder up through the plenum and out the flow wells. While this device is well suited for selective soldering, it can only solder a single batch, usually one board, at a time.
Wave soldering is the soldering method of choice for circuit boards which require through hole parts. This is a continuous process in which circuit boards are passed over a wave of molten solder. The solder wicks up through the vias and the through holes containing the component leads to consummate the soldering process. While this process lends itself to automation as the parts are simply loaded onto a conveyor which in turn passes the parts over a wide wave of solder, it also imposes restrictions in terms of which parts can be exposed to the high thermal shock of a molten solder wave. For instance, it is often times desirable to mix surface mount components and through hole components on the same board. However, it usually isn't desirable to expose a surface mount component directly to a solder wave as thermal damage to the component may result. Additionally, the connections of surface mount components are so close together that solder can easily bridge two or more connections. Consequently, boards containing components or areas on the side of the board which cannot be exposed to a wave, which otherwise would be wave soldered, must be soldered by some other method.
U.S. Pat. No. 5,148,961 to HUMBERT ET AL. discloses a partial solution to solve this problem which is a hybrid soldering technique. The technique disclosed creates a narrow solder wave from a solder pot and selectively applies this wave to a circuit board through a pallet containing a solder mask for the narrow portion of the board which is exposed to the solder pot wave. The pallet mask includes through holes which correspond to the areas of the board on which solder is to be deposited and recesses in the mask to house and protect components which would otherwise be exposed to the solder. The pallet is also provided with an inverted "V" shaped channel on the pallet's underside to prevent solder from splashing onto other parts of the circuit board. Unfortunately, this solution still requires a separate soldering device apart from a wave soldering machine. Also, this solution is not suited or capable of simultaneously and selectively soldering two or more areas of a circuit board which are not located in the same narrow area of the board, since the entire board cannot be simultaneously passed through the wave.
What is needed is a way to selectively apply solder to a workpiece, such as a circuit board, using a standard or conventional wave soldering machine. Some of the advantages realized by satisfying this need include: simultaneously and selectively soldering areas of a workpiece which are spaced apart relatively large distances; employing existing wave soldering machinery already in the possession of the manufacturer to wave solder pieces which previously could only be soldered by other less efficient or desirable methods and realizing the aforementioned advantages with only a small investment in additional tooling.
Objects of this invention therefore include the provision of a reusable shield for protecting selected areas of a workpiece from molten solder and heat as the shield and workpiece are passed through a solder wave. An additional object of this invention is to provide a low cost solution to some the aforementioned disadvantages of the prior art. Still another object of the invention is to provide a solution which can be used with existing wave soldering devices to selectively apply solder to a workpiece and thereby realize all of the advantages of wave soldering for pieces which previously could not be wave soldered.
Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.